The present invention relates generally to a mobile marine structure of the type having a floatable hull which can be jacked up on the legs of the structure so as to serve as a stable working deck, said deck being provided with movable cantilever beams for supporting drilling and other equipment thereon at a location outside the periphery of the deck. Commonly referred to as "cantilevered jack-up rigs", such structures are often used to drill or service wells which are associated with and located near fixed production platforms.
Fixed production platforms, hereinafter "platforms", are generally erected to facilitate the production and transmission of oil and gas from wells that have already been successfully drilled. Many platforms are not equipped with the heavy machinery necessary to drill new wells or service existing wells. Hence, if it becomes necessary to service the existing wells of such a platform, or if it is desireable to drill new wells so as to enhance the production to the platform, the necessary servicing and drilling equipment must be brought to the platform site.
Such drilling and servicing equipment is sometimes provided in the form of a platform drilling rig. A platform drilling rig is typically comprised of several large, heavy and unitized packages, including a drilling unit, engine unit, crew quarters unit, mud pump units, liquid mud storage unit, drilling water storage unit and bulk material storage unit. In use, the unitized packages of a platform drilling unit are transported to the platform via a barge. The units are transferred from the barge to the platform, and the drilling and servicing operations are then conducted from the platform. When the operations have been completed, the unitized packages are transferred from the platform back to the derrick barge.
There are several disadvantages associated with the above described use of a platform drilling rig, including the need for a relatively large amount of deck area on the platform. Not all platforms have a deck area which is big enough to accommodate the large and numerous unitized packages of the platform drilling rig. Additionally, the installation of a platform rig is a very time consuming and costly process. Moreover, the heavy unitized packages of the platform drilling rig impose a tremendous vertical load on the fixed production platform which may exceed or come dangerously close to the platform's design load capacity.
As an alternative to the use of a platform drilling rig for the above described purposes, a cantilevered jack-up rig is sometimes employed. In use, a cantilevered jack-up rig, with its hull floating in the water, is self-propelled, towed or otherwise moved to the location of the subject platform and is positioned as close to the platform as is operationally required. The legs of the cantilevered jack-up rig are then lowered to the seabed and the hull is jacked up on the legs, out of the water and above the reach of the sea waves, where it serves as a stable work deck from which well drilling, servicing and other activities can be conducted.
The hull/deck of the cantilevered jack-up rig is typically equipped with and provides support for a pair of cantilever beams which may be skidded or otherwise moved laterally with respect to the hull/deck. Such lateral movement of the beams is typically in a longitudinal direction, i.e., forward and aft. The cantilever beams support and laterally transport drilling and servicing equipment, including a derrick. After the hull/deck of the cantilevered jack-up rig has been jacked up a suitable height above the water, the cantilever beams are skidded laterally across the hull/deck such that the unsupported or "free" ends of the cantilever beams, with the derrick supported thereon, extend laterally beyond the periphery of the hull/deck. From such a cantilevered position, the derrick can service and/or drill the wells associated with the platform, including wells located directly below the platform.
The cantilevered jack-up rig has an advantage over the platform drilling rig in that the former does not involve the placement of any equipment onto the platform. Therefore, the use of a cantilevered jack-up rig is not limited by the space and weight considerations which apply to the use of a platform drilling rig.
However, the cantilevered jack-up rig has other limitations which detract from its usefulness. More specifically, the vertical load capacity of the free end of the cantilever beam reduces in proportion to its unsupported length. Hence, there is a practical limitation on the lateral extension or "reach" of the cantilever beams and the equipment they support. Due to such reach limitation, a cantilevered jack-up rig may be unable to service or drill all of the desired platform wells from a single position. In order to service or drill such "unreachable" wells, the operations of the cantilevered jack-up rig must be interrupted so that the hull/deck can be jacked down to the water, the legs raised from the seabed and the entire rig moved to a new position from which the previously "unreachable" wells may be reached. Such interruption of operations and movement of the cantilevered jack-up rig are very expensive, time consuming and may not be practically possible.
Several attempts have been made to deal with the problems associated with the limited accessibility and reduced beam load capacity of cantilevered jack-up rigs. For example, U.S. Pat. No. 4,973,198, issued to Cox, discloses a method and apparatus for transferring a drilling derrick from a jack-up rig to a fixed platform. According to Cox, the fixed platform is provided with a removable upper platform section having a suitable size to accommodate the drilling derrick to be positioned thereon. The removable upper platform is provided with a series of power jacks or other hoisting mechanisms such that the upper platform section becomes a leveling unit which is maintained in an appropriate horizontal alignment with the jack-up drilling rig.
One of the obvious disadvantages with Cox is that it requires a potentially huge investment of time and money in order to install the upper platform section onto the fixed production platform. Another problem with Cox it that it involves the imposition of the entire weight of the drilling derrick onto the fixed production platform.
Another attempt at solving the general problem is disclosed in U.S. Pat. No. 4,938,628, issued to Ingle. Ingle discloses an apparatus and method for transferring a drilling module from a jack-up rig to a fixed platform. The jack-up rig is provided with a cantilever beam assembly which supports a drilling module. The cantilever beam assembly, with its supported drilling module, is extended to a position over the fixed platform. The hull of the jack-up rig is then jacked down until the drilling module rests on the fixed platform, allowing the cantilever beam assembly to then be withdrawn and returned to the jack-up rig. Whatever its merits, Ingle does not solve the problem associated with transferring the entire vertical load of the drilling module onto the fixed platform.